Noise cancellation system and headphone therefor

ABSTRACT

A noise cancellation system includes a headphone having a sound transducer and a headphone speaker. Noise cancellation circuitry is provided remote from the headphone and supplies the headphone speaker and is supplied from the sound transducer. A filter normalizes the output from the sound transducer of the headphone to the noise cancellation circuitry.

This application claims priority from PCT/NZ2003/000134, entitled NOISECANCELLATION SYSTEM AND HEADPHONE THEREFOR, filed Jun. 26, 2003, whichclaims priority from New Zealand Patent No. 519,863, entitled NOISECANCELLATION SYSTEM AND HEADPHONE THEREFOR, filed Jun. 28, 2002, both ofwhich are incorporated herein by reference.

I. BACKGROUND OF THE INVENTION

A. Field of Invention

This invention relates to noise cancellation systems, and is directedparticularly, but not solely, towards a headphone noise cancellationsystem.

B. Description of the Related Art

It is known to provide noise compensation systems with either noisecancellation built into the headphone or with a fixed noise cancellationsystem forming part of the system into which a headphone is plugged.U.S. Pat. No. 5,182,774 exemplifies the former type which have the noisecancellation tailored to each headphone type. The latter have previouslybeen designed for use with only one manufacture of headphone and thenoise cancellation is problematic with other types because of variationsin frequency response and impedance which render the cancellationineffective and can, at worst, result in positive feedback andinstability.

II. SUMMARY OF THE INVENTION

According to one aspect of this invention, a noise cancellation systemincludes: a headphone including a sound transducer and a headphonespeaker; noise cancellation circuitry provided remote from theheadphone, supplying the headphone speaker, and being supplied from thesound transducer; and, a filter to normalize the output from the soundtransducer of the headphone to the noise cancellation circuitry.

According to another aspect of this invention, the filter may be locatedat the output of the sound transducer to enable effective noisecancellation to be achieved in use.

According to another aspect of this invention, the filter may comprise apassive electronic filter.

According to still another aspect of this invention, the filter maycomprise a resistor/capacitor network.

According to another aspect of this invention, the filter may be a highpass filter in parallel with the sound transducer.

According to another aspect of this invention, the sound transducer maycomprise an electret condenser microphone.

According to yet another aspect of this invention, a headphone for anoise cancellation system includes: one or more headphone speakers forproviding sound to a user; at least one sound transducer provided in theheadset adjacent to the speaker; wherein the output of the soundtransducer is provided as an electrical signal and is provided to afilter; and, wherein the output of the filter is available to noisecancellation circuitry to cancel noise from the signal being deliveredto the speaker.

It is one object of the present invention to provide an improved noisecancellation system.

It is another object of the present invention to provide an improvedheadset for a noise cancellation system.

It is yet another object of the present invention to provide the publicwith a useful choice.

Various benefits and advantages of the invention will become apparent tothose skilled in the art to which it pertains upon a reading andunderstanding of the following detailed specification.

III. BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement ofparts, a preferred embodiment of which will be described in detail inthis specification and illustrated in the accompanying drawings whichform a part hereof and wherein:

FIG. 1 is a circuit schematic of a noise cancellation system accordingto the invention.

FIG. 2 is a schematic of an equivalent circuit for a headset microphoneand filter according to the invention.

FIG. 3 is a schematic illustrating one example of a practicalimplementation of the invention.

FIG. 4 is a perspective view of a set of headphones.

IV. DESCRIPTION OF THE PREFERRED EMBODIMENT

In general terms, the invention provides a way of allowing noisecancellation circuitry which is typically provided remote from aheadphone, to be able to provide effective noise cancellation for anumber of different headphone designs. This is achieved by providing thenoise cancellation headphone sound transducer (typically a microphonesuch as an electret condenser microphone) with a passive filter so thatthe feedback signal provided by the microphone is appropriatelyconditioned for a “generic” active noise cancellation circuit ornormalized. Therefore, the invention allows the noise cancellationcircuitry to be designed to be operative over a certain phase range ofinput feedback signals from a headset. This in turn means that a filterplaced on the headset feedback signal may be appropriately configuredfor each different sort of headset so as to be acceptable to the noisecancellation circuitry and enable collective noise cancellation to beachieved.

The most preferred form of the invention the filter comprises a simplepassive filter. Most preferably it is a resistor/capacitor filter asdescribed further below. We have found that simple resistor capacitorpassive filter provides an appropriate transfer function that issuitable for active noise cancellation applications. This simple passivefilter may have the values of resistance or capacitance varied dependentupon the nature of the headphone, the headphone sound transducer etc.

Turning now to FIG. 1, a schematic of an implementation of the inventionas shown. The headphone is shown to the left of dashed line 2 in thefigure and is generally indicated by arrow 4. On the other side of thedrawing, i.e., to the right hand side of dashed line 2, the noisecancellation circuitry is shown generally referenced 6. By way ofexample, the noise cancellation circuitry may be provided in a portableelectronic device such as a portable audio system including those soldunder the trade mark WALKMAN. The circuitry could alternatively beprovided in a home stereo system, television set or a variety of otherdevices which provide sound to a user. However, more typically, thenoise cancellation circuitry 6 will be provided in a passenger vehicle.Again, a number of different forms of passenger transport may beprovided so the noise cancellation circuitry may be provided in a seatinstallation (possibly an arm rest area) of a commercial aeroplane, atrain a bus, a private automobile, or the like.

In FIG. 1, the sound transducer for the headphone 4 is an electretcondenser microphone 8, and the output of the microphone is provided toa passive filter network comprising resistor 10 and capacitor 12. Theoutput from the passive filter network is referenced 14 and 16, andthese outputs are typically provided as pins on a plug which isacceptable to an appropriate jack or socket on the device that includesthe noise cancellation circuitry.

Still referring to FIG. 1, the headset 4 also includes a speaker 18which has input signal connections 20 and 22. Again, connections 20 and22 are in use electrically connected to an appropriate plug pins (notshown) so that they can be supplied with an appropriate electricalsignal from the output of the noise cancellation circuitry that isprovided in the corresponding socket remote from the headphone.

Turning now to the noise cancellation circuitry, the input to the noisecancellation circuitry from the microphone is represented by inputs 24and 26. Input 26 may be a reference such as ground, input 24 is providedto an amplifier 28 via capacitor 30. The power supply VCC and biasresistor 32 are also provided. The output of amplifier 28 is fed tonoise cancellation circuitry which may comprise a passive network or beactive, for example being implemented using a microprocessor. Noisecancellation circuitry which may be used is not described in thisdocument, as it is known to those skilled in the art. The output fromthe noise cancellation circuitry is provided to an appropriate outputamplifier 34 to be provided to output terminals 36 and 26 which connectto terminals 2 and 22 for the headphone speaker.

Turning now to FIG. 2, further explanation of the passive filter networkdescribed above is illustrated. The sound transducer 8 in the preferredform of the invention comprises an electret condenser microphone. Thismicrophone behaves as a current source from a signal viewpoint. Using aNorton to Thevenin conversion the microphone signal can be representedas a voltage source in series with the bias resistor, Rbias. In FIG. 2,the microphone signal is represented as voltage source 40, the output ofwhich is in series with a bias resistor Rbias. The voltage source andbias resistor are in parallel with the passive filter comprisingresistor 10 and capacitor 12. The network shown in FIG. 2 provides atransfer function which is:

$\frac{1 + {sRC}}{1 + {{s( {R + R_{bias}} )}C}}$This is a suitable transfer function for active noise cancellationapplications, i.e., the output from the circuit shown in FIG. 2 isappropriate for provision to a “generic” active noise cancellationcircuit. Therefore, it can be seen that the values of resistance andcapacitance of components 10 and 12 of FIG. 2 may be chosen dependentupon the general acoustic properties of the headphone e.g., shape andsize of the ear piece and orientation of the microphone relative to thespeaker.

Finally, in FIG. 3, a typically implementation is illustrated. Thereference numerals used in this figure are the same as those used withreference to FIG. 1 and it can be seen that resistor 10 has value of 3.3kΩ, capacitor 12 is 68 nanofarads, and the bias resistor 32 is 4.7 kΩ.Typically the headphone compensation is determined by targeting thenoise cancellation at the best headphone to be catered for and thencompensating other headphones to bring them to the same performancelevel.

From the foregoing, we see that the invention provides significantadvantages in that a number of different headphones or headsets may beused to provide noise cancellation without having to redesign, alter ormodify noise cancellation circuitry for which they are used. Therefore,noise cancellation circuitry may be designed to standard parameters andembodied in various devices such as personal stereos or passenger seatinstallations while allowing users to use their own preferred headset,or a variety of different headsets.

The preferred embodiments have been described, hereinabove. It will beapparent to those skilled in the art that the above methods mayincorporate changes and modifications without departing from the generalscope of this invention. It is intended to include all suchmodifications and alterations in so far as they come within the scope ofthe appended claims or the equivalents thereof.

Having thus described the invention, it is now claimed.

1. A noise cancellation system comprising: a noise cancellation circuithaving a noise cancellation input to receive a noise cancellation inputsignal required for effecting noise cancellation, and a noisecancellation output for providing an output signal processed to cancelnoise dependent on the noise cancellation input signal, the noisecancellation circuit being operative over a predetermined phase range ofnoise cancellation input signal supplied to the noise cancellationinput; a plurality of headphones provided remote from the noisecancellation circuit, each headphone having a headphone speaker and asound transducer, at least one headphone of the plurality of headphoneshaving a different acoustic property from the other headphone(s) suchthat the sound transducer does not provide a noise cancellation inputsignal within the predetermined phase range, and the at least oneheadphone having a passive filter provided to filter the output of thesound transducer, and each headphone being individually electricallyconnectable to the noise cancellation circuit so that the output of thepassive filter or the output of the sound transducer is provided to thenoise cancellation input and the noise cancellation output signal isprovided to the headphone speaker; and, wherein the passive filter forthe at least one headphone is configured to modify the output of thesound transducer to provide a noise cancellation input signal which iswithin the predetermined phase range.
 2. A noise cancellation system asclaimed in claim 1 wherein the filter is located at the output of thesound transducer to enable effective noise cancellation to be achievedin use.
 3. A noise cancellation system filter as claimed in claim 2wherein the filter comprises a passive electronic filter.
 4. A noisecancellation system filter as claimed in claim 3 wherein the filtercomprises a resistor/capacitor network.
 5. A noise cancellation systemfilter as claimed in claim 1 wherein the filter is a high pass filter inparallel with the sound transducer.
 6. A noise cancellation system asclaimed in claim 1 wherein the sound transducer comprises an electretcondenser microphone.
 7. The noise cancellation system as claimed inclaim 1 wherein the noise cancellation circuit comprises amicroprocessor.